Isotope diagenesis and palaeofluid movement, Middle Jurassic Brent sandstones, North Sea

The Middle Jurassic deltaic Brent Group sandstones, northern North Sea, have a complicated diagenetic sequence which may be simplified to kaolinite - Fe,Ca carbonates - quartz overgrowths - Fe,Mg carbonates - illite. High porosities and permeabilities have existed in the Etive, Ness and Tarbert Formation sandstones throughout diagenesis. By contrast, the highly micaceous and relatively finer grained Rannoch Formation sandstones have good porosities but very poor permeabilities due to burial compaction and carbonate cementation. Oxygen and hydrogen isotope studies indicate that early diagenesis occurred in a dominantly meteoric pore water (δ¹⁸0 = -7%.). The early diagenetic cements of siderite, vermicular kaolinite and calcite started to precipitate at 14, 26 and 32°C respectively. With the onset of burial, below 1.2 km, the Brent sequence eventually became sealed off from the meteoric 'head' by Lower Cretaceous sediments. Blocky kaolinite precipitated and by the end - Cretaceous quartz overgrowth formation from a warm, dominantly low salinity water (1 - 5 wt.% eq. NaCl). Homogenisation temperatures range from 73 to 131°C. Illite preparation is cogenetic with the latest stages of quartz overgrowth precipitation (K/Ar illite date, 58Ma.) and has reduced porosity and permeability markedly in different locations prior to oil migration. The depth of burial at which this last cementation event occurred is 2.3 km. Fluid inclusion microthermometry indicates that quartz overgrowth and latest ankerite precipitation occurred in a geothermal gradient of 70°C/km. After this heat excursion the reservoirs have cooled back to present day temperatures of 85 - 115°C. Water values computed from the mineral cements indicate precipitation from a porefluid which has gradually evolved isotopically to its present day composition (δ¹⁸0 = -7 to +2%.SMOW) in an isotopic system that has become closed during burial. However the quartz and ankerite suggest one unusual episode of open system hot fluid input.The Middle Jurassic deltaic Brent Group sandstones, northern North Sea, have a complicated diagenetic sequence which may be simplified to kaolinite - Fe,Ca carbonates - quartz overgrowths - Fe,Mg carbonates - illite. High porosities and permeabilities have existed in the Etive, Ness and Tarbert Formation sandstones throughout diagenesis. By contrast, the highly micaceous and relatively finer grained Rannoch Formation sandstones have good porosities but very poor permeabilities due to burial compaction and carbonate cementation. Oxygen and hydrogen isotope studies indicate that early diagenesis occurred in a dominantly meteoric pore water (δ¹⁸0 = -7%.). The early diagenetic cements of siderite, vermicular kaolinite and calcite started to precipitate at 14, 26 and 32°C respectively. With the onset of burial, below 1.2 km, the Brent sequence eventually became sealed off from the meteoric 'head' by Lower Cretaceous sediments. Blocky kaolinite precipitated and by the end - Cretaceous quartz overgrowth formation from a warm, dominantly low salinity water (1 - 5 wt.% eq. NaCl). Homogenisation temperatures range from 73 to 131°C. Illite preparation is cogenetic with the latest stages of quartz overgrowth precipitation (K/Ar illite date, 58Ma.) and has reduced porosity and permeability markedly in different locations prior to oil migration. The depth of burial at which this last cementation event occurred is 2.3 km. Fluid inclusion microthermometry indicates that quartz overgrowth and latest ankerite precipitation occurred in a geothermal gradient of 70°C/km. After this heat excursion the reservoirs have cooled back to present day temperatures of 85 - 115°C. Water values computed from the mineral cements indicate precipitation from a porefluid which has gradually evolved isotopically to its present day composition (δ¹⁸0 = -7 to +2%.SMOW) in an isotopic system that has become closed during burial. However the quartz and ankerite suggest one unusual episode of open system hot fluid input